Physiology of Salt Stress in Plants. Группа авторов

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СКАЧАТЬ plants are being cultivated for the food, fodder, fuel production purposes, and landscaping purpose (Panta et al. 2014, 2016). Salicornia is a halophyte from the Amaranthaceae family, which grows in the temperate and subtropical coastal areas. The plants from this genus are being used to produce the cooking oil, biomass with high fiber contents, and fresh vegetable products (Glenn et al. 1991). However, in India, only one species represent the genus Salicornia named as Salicornia brachiata Roxb. (Chaudhary et al. 2018). In the coastal salt marshes of Gujarat, India, the S. brachiata is a dominant halophyte. It serves as a model plant for the study and understanding of the salt tolerance mechanisms in the plant. The S. brachiata serves as the source of several novel abiotic stress‐responsive genes (Mishra and Tanna 2017; Singh et al. 2016) which are shown to improve the salinity tolerance in the glycophytes through transgenic approach and novel stress‐responsive alternative promoters for the genetic engineering of the crops (Tiwari et al. 2019). Traditionally, the S. brachiata at their early developmental stages are used to prepare vegetable salad and pickles, and the seed oil is rich with the polyunsaturated fatty acids (Chaudhary et al. 2018). The S. brachiata can accumulate salt up to 40% of the dry biomass (Chaudhary et al. 2018), suggesting S. brachiata as a tool for the phytoremediation of the land degraded by the salinity. The halophytic shrub Atriplex lentiformis has been cultivated as forages, revegetation of the salinity‐degraded land, and rangeland enhancement programs across the globe (Jordan et al. 2009).

      Halophytes to be consumed as food, are getting popularity, and the best example is the halophyte quinoa (Chenopodium quinoa), which can tolerate the 40 dS/m of salinity. The quinoa is gluten‐free and rich in vitamins and minerals, which helped it gain popularity even in countries that are not affected by the salinity (Panta et al. 2014). The halophyte Atriplex triangularis can be grown in the soil salinity of 30 g/l with the yield potential of 21.2 t/ha fresh weight (Gallagher 1985). The taste of leaves from the A. triangularis is similar to the spinach. Therefore, it is a component of the human food in European countries like the Netherlands, Belgium, and Portugal (Panta et al. 2014). The use of halophytes grown in the wasteland or salinity‐affected land as forage may reduce the load on agricultural land and freshwater resources for cultivation of forage. However, the candidate halophyte selection depends on their biomass productivity, nutrient quantity, and quality, which should not affect livestock productivity. The halophytes saltbush (Atriplex spp.) and blue bush (Maireana spp.) are being cultivated in salt‐affected coastal areas and used as a forage crop for feeding the animals. Several halophytes were grown successfully using the diluted seawater irrigation practices and feeding the cattle of the poultry and meat industry without affecting the quality and productivity of the meat (Panta et al. 2014).

Production of halophytes as the oilseed provides another dimension of the use of the salinity‐degraded land and gain alternative agriculture practices in the salt‐stress conditions. Several species from the genus Salicornia, Suaeda, and other halophytes showed no accumulation of the salt in the seed, no matter what saline conditions they were grown (Panta et al. 2014). The Salicornia bigelovii showed the higher yield potential of the seeds, even at the soil salinity equivalent to 70 g/l NaCl concentration (Glenn et al. 1999). When the S. bigelovii growing area was irrigated with the seawater having salt concentration equivalent to 40 g/l, it was able to produce seed at a rate of 2 t/ha (Glenn et al. 1999). This yield potential is equivalent to the oilseed production from the conventional crop in the nonsaline conditions (Panta et al. 2014). It must be noted that S. bigelovii not only has the higher yield potential in the salt‐affected areas but its seed is also rich in the oil (30%) with similar properties to the oil derived from the safflower (Glenn et al. 1991; Zerai et al. 2010) and protein (35%) content (Panta et al. 2014). These findings provide hope to the farmers across the globe whose agricultural land is degraded due to the salinity and can help us minimize demand load on the glycophytic crop to increase their productivity to feed the growing population. The higher oil yield potential of the halophyte seed opens the opportunity to use the oil produced from the halophytes for biodiesel production without competing with oilseed crops used for food. In a trial at the Mexico coast for oilseed productivity, the S. bigelovii showed higher seed productivity in seawater irrigated fields than the other oil seeds, such as the mustard, safflower, and sunflower grown under nonsaline conditions (Glenn et al. 2009; Panta et al. 2014). The perennial halophytic Tamarix species can grow and survive in drought and flooding conditions and is salinity tolerant (Panta et al. 2014). The plant has very high yield potential ranging from 19.5 to 52 t/ha/year and heat value relative to standard coal at the ratio of 1/0.7 (Panta et al. 2014). Higher heat value and biomass yield potential make it an important halophytic candidate for the thermal electricity generation and lignocellulosic second generation biofuel production. The cooperation of Boeing, Etihad Airways, and UOP Honeywell with the Masdar Institute of Science and Technology to grow Salicornia species at the Abu Dhabi for production of aviation industry‐grade biofuel can exemplify the importance of biofuel production using the halophytes (Panta et al. 2014).

      The unique ability of halophytes to accumulate salt ions in the root and shoot vacuole with very low leakage provides them the unique ability to extract salts from the soil. It thus may act in the desalinization of degraded land. Some halophytes such as S. brachiata (40% of dry weight) and Atriplex sp. (39% of the dry weight) serve as the salt accumulator (Barrett‐Lennard 2002) and therefore can be extensively used for the phytoremediation. Panta et al. (2014) have provided a table of the halophytes from the previously published report, which describes different species from the genus Suaeda, Atriplex, Tectocornia, Sesuvium, Anthrocnemum, Excoecarcia, Ipomoea, Batis, and Salicornia, which were used in past or have a high potential in coming future for the desalinization and removal of the heavy metals from the degraded soil.

2.7 Conclusion and Future Perspectives

      Salt stress affects plants’ survival and productivity. Research on understanding the mechanism of salinity stress gave us significant insight into the plants’ response to salt stress and how halophytes respond differently than the glycophytes. However, the complete mechanistic understanding of the salt tolerance mechanism in the halophytes at the developmental and intracellular molecular levels is lacking. To meet the food, feed, fiber, and fuel demand of the growing human population and improve the abiotic stress resilience of the crop, there are two ways to pursue in parallel; one, adopting the halophyte for food, forage, and fuel generation by growing them in the salt‐affected land, and, second, understanding the salt‐stress tolerance mechanism in the halophytes and then engineering the genome of glycophytic crop plants for better salt‐stress tolerance. Halophytes grown by seawater irrigation have shown their significance by producing a higher yield of oil and protein‐rich seeds than a conventional oilseed crop, which suggested that the salt‐degraded land can be used for growing halophytes for edible oil or biofuel. Halophytes were also used to restore the wasteland by desalinization and phytoremediation of the heavy metals, used as forage for the cattle and vegetable salads in different parts of the world. The halophyte C. quinoa seeds are rich in minerals and vitamins and gained popularity as mainstream food because of its gluten‐free nature. For improving the salt‐stress tolerance in the crop plants, the current requirement is to understand salt‐stress sensory system at the root, root to shoot signaling, and intracellular sensory and responses at the cell organelles such as chloroplasts, mitochondria, or peroxisomes in halophytes. In halophytes, yet very little is known about the development and functioning mechanism of the EBC or different types of the salt glands. Understanding these mechanisms and engineering, the crop using the genome‐editing approach may improve the salt‐stress tolerance of the crops within a shorter time frame.

References

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